In many cases the useful life of vegetable cooking oil can be extended by removing sedimentary solids from the oil. Many components, e.g., salt or eggs, of product coatings, whether batter or breading, can cause oil problems such as accelerated free fatty acid build-up. In additiion, charred particles can transfer a char taste to the oil, and very fine charred particles may give a premature charred color to the oil. The solution to these problems is removal of the sedimentary solids, whether crumbs or finely divided suspended particles, before these problems occur.
If suspended particles are allowed to accumulate in a fryer, black specks may appear on the surface of the product. It is a universal standard among food processors that these specks do not appear on any product. The solution to this problem is particle removal by filtration before the problem can occur.
Another area of concern is the fact that crumb accumulation, wherein heavier particles settle out of the oil, can adversely affect the performance of the fryer. It is interesting that this is true whether the fryer is directly or indirectly heated. With either heating method, crumb accumulation can adversely affect heat transfer.
In one type of directly heated fryer, wherein tubes passing through the oil may be gas fired or contain electrical heating elements, crumbs accumulating on the bottom of the fryer may build up and envelop the immersed tubes so that heat transfer from the tubes to the oil is decreased and oil is not heated adequately for frying. At the same time, tubes may overheat and burn through, a dangerous situation due to fire hazard. Another tube of fryer may have a heat transfer medium circulating through the tubes. This type is vulnerable to lack of heat transfer, but not to burnout. Finally, the seldom used type of fryer incorporating direct firing on a nominally flat bottom is especially vulnerable to both low heat transfer and burnout. It is important to note that some deep fat fryers have been available with a mechanism to remove any solids which settle to the bottom of the fryer, but even this mechanism cannot remove suspended solids.
In fryers which employ a remote heater and circulate the frying oil from the heater to the fryer continuously, if the circulation tubes in the heater suffer an accumulation of sediment, heat transfer will be reduced and fryer performance decreases. In addition, sediment can accumulate in transfer piping, particularly at elbows, to the point where the fryer is completely inoperative. Once again, continual removal of sedimentary solids solves the problem.
Over the years several approaches have been developed to filter frying oil. The simplest of all is the sediment basket. This unit consists of a tank into which is inserted a "basket". This basket consists of a framework supporting a wire screen which does the filtering. Oil enters the top of the basket, the crumbs are trapped by the wire screen and the semi-clean oil is pumped out of the bottom for re-use. Although this system is very simple, it is objectionable. First, the hot oil is circulated over the collected crumbs, normally for the length of the work shift, the crumbs continue to cook and char. The oil is circulating through the sediment basket is continually exposed to these charred crumbs and picks up a taste and a dark color. Second, the oil level in the circulation tank may be above the basket at the end of the shift. Thus, the sediment may spill over the basket and not be filtered. Third, the wire screen used must have openings large enough so that they do not plug too rapidly, thus the filtering action cannot trap a very large proportion of the suspended fines. These continue to circulate with the detrimental effects noted above, oil discoloration and black specks. When raised from the tank for emptying, the hot sediment in the basket can be very dangerous to personnel.
Another filter applied to fryer oil filtration is the plate and frame filter which employs filter paper inserted between each of a series of plates and frames. Every work shift or more frequently, the filter must be disassembled, the filter paper being discarded along with the sediment collected -- a time consuming operation. Then the plates and frames are cleaned before the filter can be reassembled, and the filter paper inserted again.
Often fine particles tend to block these filters rapidly, in as little as one or two hours, rendering the filter inoperative until cleaned. When the filter is blocked up during the work shift, it is necessary to disassemble it, clean out the plates, reassemble the filter and insert the paper. This procedure takes 20 to 30 minutes. Unfortunately, during this time when the filter is out of operation, the piping lines can become plugged. These filters have the disadvantage of running the hot frying oil over the collected sediment, as with the sediment basket.
To eliminate the exposure of the frying oil to the sediment collected on the filter during a work shift, a filter which replaces the active filtering surface on demand came into use. In this style of filter a roll of filter paper is mounted at one end of the unit. The end of this paper is then threaded over a conveyor belt and out the filter discharge. The paper carrying conveyor belt is designed so that a shallow pocket is formed in the center with high edges at conveyor belt sides and at support rolls, with all high edges essentially horizontal. The filter paper conforms to the pocket shape. Hot dirty oil is deposited in the pocket, the oil passing through the paper while sediment is collected on the paper. As sediment collects, it becomes more and more difficult for the oil to pass through the paper, and the level of oil rises. Eventually, the oil rises to a height in the pocket where a level sensor is activated. Then the filter paper with the collected sediment is indexed out of the filter and clean filter paper is introduced into the active filter area. The filtering action may or may not be interrupted during the paper change cycle. The sediment and dirty paper are collected in a container for disposal. Because the oil passes through the paper by gravity, hence with very little pressure, the sediment collected on the paper is neither thick nor compact. Since a pocket in the center is formed for filtering, the edges of the paper cannot be utilized. Also, fine particles of sediment tend to block off the paper very quickly. These factors combine to make for high paper usage. Usage of one to three rolls per shift, each roll 250 yards long, is not unusual. As with the plate and frame filter, there will always be the expense of filter paper and the loss of oil absorbed by that paper. At today's prices employing this style filter can cost the operator very dearly.
One other style of filtration has come into use over the past few years. This type of filter is called the vibratory separator. Basically, what happens with the separator is that oil is dropped from above onto a vibrating screen. Ideally, the oil passes through the screen and the crumbs vibrate to a discharge position for collection. However, to function properly for a particular type of crumb, the screen must be balanced one way. To handle another type of crumb -- say the operator changes from standard breading to flour, the balance must be reset. This separator seems to handle "hard" crumbs fairly well, like those from standard breading, but "soft" crumbs such as might come from other material are more difficult to vibrate successfully. Oil losses are very high with this type of filter. The frying oil is aerated while dropping onto the vibrating screen and is further aerated by the vibrating screen. Aerating can be very detrimental to oil quality by causing more rapid oxidation thereof.
The filter of this invention incorporates a continuous fine mesh stainless steel wire belt, sometimes called filtercloth, through which the oil is circulated. The fine mesh wire belt catches the particles, and the cleaner oil is returned to the cooking operation. As particles collect on the belt, oil level increases, and at a predetermined level, the belt is indexed to present a cleaner area of belt over the active filter area. During indexing, the sediments stripped from the belt and collected are disposed of. The oil flow through the belt is assisted by suction created by a positive displacement pump. This suction provides several advantages. The height of the sediment collected is considerably higher than in the gravity type filter above, thus the particles themselves are put to work as a layer of filtering medium, filtering smaller and smaller particles. The suction aids in compacting the collected sediment, thus reducing the amount of oil trapped within the sediment when the sediment cake is stripped from the belt.
Perhaps the most important feature in this unit is that for most operations no filter paper is required. Thus, a large expense is eliminated. In addition, expense of the oil lost in absorption by filter paper is eliminated.
For those operations which do require filter paper, a provision for paper usage in an additional paper layer may be incorporated into the filter. For these applications paper usage will be less than that required on a gravity flow type, because of the suction feature and full utilization of paper width.
Further, the oil is introduced below the surface of the oil above the filter belt. As a result, any possibilities for aeration of the oil have been greatly reduced.
An object of the present invention is to provide a filter for removing the sediment in hot cooking oil by passing the dirty oil through an endless fine mesh filter belt and indexing the belt intermittently as it becomes clogged with the sediment so as to present a cleaner section of the belt over the suction opening.
Another object of the invention is to provide an economical and dependable means for driving and guiding or tracking a delicate endless fine mesh filter belt in a generally L-shape path over a suction box, past a cleaning point, and returning the belt to the starting point, either continuously or in intermittent starts and stops, while insuring long belt life. The belt and its driving and guiding means are so arranged that a belt may be easily removed and replaced by another belt, either for repair or for replacement, if necessary, with a belt of different mesh.